In the field of printing presses, such as flexographic printing presses, it is important to maintain a constant tension of a web of printing medium or material passing through a press to control the quality of an image printed on the web of medium. This is particularly true in flexographic printing presses that have multiple printing stations for applying different colors because if an image printed on the web at one printing station does not register or align with a subsequent image printed at another printing station, then the resulting composite printed image will appear blurred and possibly discolored.
It is critical to maintain a constant tension of the a web of printing medium when the printing medium is a non-elastic extensible material such as foil. Foil material exhibits non-elastic properties so that once it is stretched, for instance under varying tension levels as it passes through the printing press, the foil remains stretched which results in the registry problem mentioned above.
It is known to use a horizontal or vertical dancer roller assembly to maintain the tension of the web of printing medium passing through the printing press. A conventional dancer roller assembly includes a plurality of dancer rollers, any number of which are fixedly mounted to a frame of the printing press while the remaining rollers are slidable toward and away from the fixed rollers. The web of printing medium is fed from a mill or supply roll through the dancer rollers before being fed to the successive printing stations and taken up at a take-up station.
The position of the slidable dancer rollers relative to the fixed dancer rollers is a function of the amount of web material being fed to the printing stations in relation to the amount of web material being pulled through the printing stations by a take-up roller at a given time. When more printing material is being fed to the printing stations than can be printed by the printing stations, the dancer roller assembly takes up the excess printing material by urging the slidable dancer rollers away from the fixed dancer rollers to extend the length of travel of the web of material upstream from the printing stations. When not enough printing material is being fed to the printing stations in relation to the amount of web material passing through the printing stations, the dancer roller assembly feeds additional printing material to the printing stations by urging the slidable dancer rollers toward the fixed dancer rollers to reduce the length of travel of the web of printing material upstream from the printing stations.
In order to accurately maintain the tension of the web of printing medium as it passes through the printing press, it is necessary to maintain a constant bias on the dancer roller assembly, and more particularly on the slidable dancer rollers so that no bias fluctuations will be transmitted to the web of printing medium to cause the undesirable web tension variations. Springs have previously been used in an attempt to maintain the required constant bias on the dancer roller assembly. However, a spring generally does not provide a constant force throughout a full range of compression and extension of the spring. Thus, springs can not maintain a constant bias on a dancer roller assembly to the level required for printing on a web of non-elastic extensible printing medium.
What is needed therefore is an apparatus for maintaining a constant biasing force on a dancer roller assembly so as to maintain a substantially constant tension on a web of printing medium.